Temperature

The concept of temperature has two related, but different,
interpretations. On a general level, temperature is associated with the
sense of hot and cold. If you put your finger in a pan of hot water, heat
energy flows from the water to your finger; you say that the water is at a
higher temperature than that of your finger. If you put your finger in a
glass of ice water, heat energy flows as heat away from your finger.

The direction of heat energy flow is the basis of one definition of
temperature. Temperature is the property of objects—or more
generally of systems—that determines the direction of heat energy
flow when the objects are put in direct contact with each other. Energy
flows as heat from objects at a higher temperature to ones at a lower
temperature. When heat energy ceases to flow, the objects are at the same
temperature and are said to be in thermal equilibrium.

The second definition of temperature is more rigorous. It deals with the
factors that are responsible for an object's being warm or hot on
the one hand or cool or cold on the other. This definition is based on the
behavior of the particles (atoms, molecules, ions, etc.) of which matter
is made. On this level, temperature can be defined as the total kinetic
energy of the particles of which a material is made.

Kinetic energy is the motion of particles. Particles that are rotating
rapidly on their axes, vibrating back and forth rapidly, or traveling
rapidly through space have a large amount of kinetic energy. Particles
that are moving slowly have relatively little kinetic energy.

Words to Know

Absolute temperature scale:
A temperature scale that has the lowest possible temperature—at
which all molecular motion ceases—set at zero.

Absolute zero:
The lowest possible temperature at which all molecular motion ceases.
It is equal to −459°F (−273°C).

Kinetic energy:
Energy of an object or system due to its motion.

Pyrometer:
A device for obtaining temperature by measuring the amount of radiation
produced by an object.

Resistance thermometer:
A device for obtaining temperature by measuring the resistance of a
substance to the flow of an electrical current.

Thermometer:
A device for obtaining temperature by measuring a temperature-dependent
property (such as the height of a liquid in a sealed tube) and relating
this to temperature.

From this perspective, a glass of warm water has a high temperature
because the molecules of the water are moving rapidly. The molecules of
water in a glass of cool water, by comparison, are moving more slowly.

Temperature measurement: Thermometers

Thermometers are devices that register the temperature of a substance
relative to some agreed upon standard. For example, a thermometer that
reads 32°F (0°C) is measuring a temperature equal to that of
ice in contact with pure water.

Thermometers use changes in certain physical or electrical properties to
detect temperature variations. The most common kind of thermometer
consists of a liquid—usually mercury or alcohol—sealed in a
narrow tube. When the thermometer is placed in contact with a substance,
heat travels into or out of the thermometer. If heat leaves the
thermometer, the sealedin liquid is cooled and it contracts (takes up less
space); the level of the liquid in the thermometer falls. If heat enters
the thermometer, the liquid is warmed and it expands; the level of the
liquid in the thermometer rises.

A resistance thermometer is based on the fact that all things resist the
flow of an electric current to some degree. Furthermore, such resistance
changes with temperature. In general, the higher the temperature of a
substance, the more it resists the flow of an electric current. This
principle can be used to measure the temperature of a substance by
observing the extent to which it resists the flow of an electric current.

Another type of thermometer is known as a pyrometer. A pyrometer is a
device that detects visible and infrared radiation given off by an object,
then converts that information to a temperature reading.

Temperature measurement: Scales

In order to establish a scale against which temperatures can be measured,
one first has to select two fixed points from which to begin.
Historically, those two points have been the boiling point and freezing
point of water. The two points were chosen because water is the most
abundant compound on Earth, and finding its boiling and freezing points is
relatively easy.

One way of making a thermometer, then, is to begin with a narrow tube that
contains a liquid and is sealed at both ends. The tube is then immersed in
boiling water, and the highest point reached by the liquid is marked in
some appropriate way. Next, the tube is immersed in a mixture of ice and
water, and the lowest point reached by the liquid is marked in a similar
way. The distance between the lowest point and highest point is then
divided into equal sections. The numbers assigned to the lowest and
highest point on the thermometer—and the form of dividing the range
between them—is what distinguishes one system of measuring
temperature from another.

In the early 1700s, for example, German physicist Gabriel Daniel
Fahrenheit (1686–1736) decided to assign to the freezing point of
water a temperature value of 32 and to the boiling point of water a
temperature value of 212. He then divided the distance between these two
points into 180 equal divisions, each equal to one degree of temperature.
The Fahrenheit (F) system of measuring temperature is still in use today
in the United States.

A somewhat more logical system of defining the temperatures on a
thermometer was suggested in 1742 by Swedish astronomer Anders Celsius
(1701–1744). Celsius suggested assigning the values of 0 and 100 to
the freezing point and boiling point of water and dividing the distance
between these two into 100 equal parts. The Celsius system is now used
throughout the scientific community and in all countries of the world
except the United States and Burma.

Both Fahrenheit and Celsius temperature scales have one important inherent
drawback: in both cases, negative temperatures can exist. The freezing
point of carbon dioxide (dry ice), for example, is −110°F
(−78.5°C). But recall the definition of temperature as a
measure of the average kinetic energy of the particles that make up a
substance. What meaning can be assigned, then, to a negative temperature
reading? There is no such thing as negative energy in systems with which
we are familiar.

To remedy this problem, a third temperature scale was invented in 1848 by
English physicist William Thomson (1824–1907). Thomson set the
lowest point on the temperature scale as the lowest possible temperature,
absolute zero. Absolute zero is defined as the temperature at which all
motion of all particles would cease, a condition in which heat would be
absent and, hence, a substance had no temperature. Theoretical
calculations suggested to Thomson that the Celsius temperature
corresponding to that condition was about −273°C
(−459°F). Absolute zero, then, was set at this temperature
and assigned the value 0 K. The unit K in this measurement stands for
Kelvin, the unit of measure in the absolute temperature system. The term
Kelvin comes from William Thomson's official title, Lord Kelvin.